Autotrophic denitrification in microbial fuel cell

AutotrophicdenitrificationinmicrobialfuelcellstreatinglowionicstrengthwatersAutotrophicDenitrificationinMicrobialFuelCellsTreatingLowIonicStrengthWatersSebastiaPuig,*,?MartaComa,?JoachimDesloover,?NicoBoon,?JesusColprim,?andM.DolorsBalaguer??LaboratoryofChemicalandEnvironmentalEngineering(LEQUiA),InstituteoftheEnvironment,UniversityofGirona,CampusMontilivis/n,FacultatdeCiencies,E-17071Girona,Spain?LaboratoryofMicrobialEcologyandTechnology(LabMET),GhentUniversity,CoupureLinks653,B-9000Gent,BelgiumINTRODUCTIONTheNitratesDirective(91/767/EU),theWaterFrameworkDirective(Directive2021/60/EC),andtheGroundwaterDirective(Directive2021/118/EC)allconsidernitratepollutiontobeoneofthemainthreatstogroundwaterquality,requiringurgentandintensivemonitoringandstrongpolicies.Groundwaterunderthedirectinfluenceofsurfacewaterisatparticularriskofcontaminationbynitratescomingfromagriculturalrunoffandwastewaterdischarges.1?3Thepresenceofelevatedconcentrationsofnitratesindrinkingwaterreservoirshasbecomeaseriousworldwideconcern,leadingto,amongotherhealthproblems,anincreasedriskofrespiratorytractinfectionsandgoiterdevelopmentinchildren.4,5Groundwaterpollutedwithnitratescontainsalmostnoorganicmatter.Asaresult,usingbiologicaldenitrificationinactivatedheterotrophicsludgesystemsrequiresthecarefuladditionofanexternalcarbonsource.6SomeEuropeancountriesstrivetowardtheproductionofhigh-qualitydrinkingwaterwithoutcreatingdisinfectionresiduals,primarilythroughthecontrolofgrowth-limitingsubstrates.7Theinabilityofdrinkingwatertosupportmicrobialproliferationisexpressedasbiologicalstabilityorbiostability.8Autotrophicdenitrificationisamoresustainablesolutionbecausecarbonisnotnecessary,andthusnocarbondioxideisreleased,leadingtobetterbiostability.Autotrophicdenitrificationcanalsobeachievedbyinorganicelectrondonorssuchashydrogenthroughelectrolysisbutrequirsahighenergyinput.9Thecathodeofmicrobialfuelcells(MFCs)canefficientlyperformautotrophicnitrateremovalwithconcomitantelectricityproduction.10,11InanMFC,organicsubstratesareoxidizedbyexoelectrogenicbacteriaintheanodiccell,whichproduceselectronsthataretransferredtoananodeelectrodeandthenflowtoacathode.Theanodeandcathodeareconnectedbyaconductivematerialthatcontainsaresistor.Protonsproducedattheanodemigratethroughtheelectrolyteacrossacationexchangemembranetoabiocathode.Bacteriaontheelectrodeserveasabiocatalyst,usingtheelectronstoreducenitratetodinitrogengas.Severalstudieshavesuccessfullyenhancednitrate/nitriteremovalinMFCs,12?14demonstratingthatnitrateornitritecanbeefficientlyremovedfromhighconductivity(2021?11000μS·cm?1)wastewaters.Conductivityismuchloweringroundwater(Revised:January11,2021Accepted:January18,2021Published:January18,2021betweentheelectrodes;15,16(ii)resistancesfromtheelectric-doublelayeranddiffusionboundarylayer;17,18(iii)slowprotontransportinsidethebiofilm;19and(iv)theresistanceofthemembranebecauseofthepHgradientbetweencompart-ments.20Toourknowledge,theuseandeffectivenessofMFCsfortheautotrophicdenitrificationofgroundwaterhasneverbeeninvestigated.Thisstudyevaluatesthepossibilityoftreatingtheselowionicstrengthwaters.Additionally,theoverpotentials(ionicloss,pHgradientsloss,transportloss,andanodicandcathodicoverpotentials)thatlimittheperformanceofMFCswereidentifiedandquantified.■MATERIALSANDMETHODSExperimentalSetup.TheMFCconsistedofananodeandacathodeplacedonoppositesidesofasinglemethacrylaterectangularchamber.Theanodeandcathodechamberswerefilledwithgranulargraphite(model00514,diameter1.5?5mm,EnViro-cell,Germany),whichdecreasedthevolumesto450mLnetanodiccompartment(NAC)and600mLnetcathodiccompartment(NCC),respectively.Theelectrodeswerepreviouslywashedin1MHCland1MNaOH.21Twothinnergraphiteelectrodes(107×6mm[anode]and130×6mm[cathode],Sofacel,Spain)wereconnectedtotheexternalresistance(25Ω)toclosetheelectriccircuit.Acationexchangemembrane(CEM,Nafion117,Dupont,USA)wasplacedbetweentheanodeandcathodeframes.Syntheticwaterwascontinuouslyfedatflowrateatthesteadystateof1.56and1.44L·d?1ineachcompartment.Aninternalrecirculationloop(105L·d?1)wasplacedineachcompartmenttomaintainwell-mixedconditions,toavoidconcentrationgradients,andtoavoidcloggingofthegranularmatrix.Thesystemwasthermostati-callycontrolledat22±1°C.InfluentCharacteristics.Theanodicfeedconsistedofnitrogen-purgedmediumwiththefollowingcharacteristics:450mg·L?1CH3COONa,152.5mg·L?1NaHCO3,9.55mg·L?1NH4Cl,340.7mg·L?1Na2HPO4·2H2O,150mg·L?1KH2PO4·2H2O,1.75mg·L?1CaCl2·2H2O,11.25mg·L?1MgSO4·7H2O,1.62mg·L?1KCl,and0.1mL·L?1microele-mentssolution(1g·L?1EDTA,1g·L?1FeSO4·7H2O,70mg·L?1ZnCl2,100mg·L?1MnCl2·4H2O,6mg·L?1H3BO3,130mg·L?1CaCl2·6H2O,2mg·L?1CuCl2·2H2O,24mg·L?1NiCl2·6H2O,36mg·L?1Na2Mo4·2H2O,238mg·L?1CoCl2·6H2O;[adaptedfrom22]).Themediumthatwasfedintocathodecontained244mg·L?1NaHCO3,100mg·L?1NaNO3,545mg·L?1NaH2PO4·2H2O,240mg·L?1KH2PO4·2H2O,2.8mg·L?1CaCl2·2H2O,18mg·L?1MgSO4·7H2O,2.6mg·L?1KCl,and0.1mL·L?1microelementssolution.Fortheconductivitytests,anadditionalsolutionwaspreparedwiththefollowingcomposition:12.2g·L?1NaHCO3,0.7643g·L?1NH4Cl,27.256g·L?1Na2HPO4·2H